The social gradient in children's mental health is well documented. What is less clear is how social inequalities influence children's development. We propose to examine this question at three levels of analysis. At the neighborhood level, we will assess ecometric properties of the local areas in which children reside and test whether neighborhood-level social processes, such as collective efficacy, mediate the effects of neighborhood poverty on children's mental health. At the family level, we will trace the effects of socioeconomic deprivation on proximal family processes (exposure to violence, food insecurity) and test how stressful early life events may become biologically embedded. At the individual level, we will use a genetically-informative longitudinal design to test the hypothesis that children's perceptions of their social position can help to explain why some children growing up in poverty do versus do not develop health problems. METHOD: We will conduct this research using data from the Environmental Risk (E-Risk) Study. E-Risk is a longitudinal study (from birth to age 12) of a nationally-representative sample of twins born in England and Wales in 1994-1995 (N=2232), selected from a registry created with the UK Office of National Statistics (ONS). We have tracked the children over the first 12 years of their lives, monitoring their development and their environmental exposures, including multiple forms of social inequality. We have gathered data about the children and their families at 6 time points (birth, ages 2, 5, 7, 10, and 12 years). We have also conducted two independent ecometric assessments of the neighborhoods in which the children are growing up, enabling us to study inequalities in children's lives at both the community and family level. INNOVATION AND SIGNIFICANCE: The proposed research has implications both for understanding and reducing health inequalities. At each level of analysis, we adopt a process approach to test how social inequalities influence children's development. At the neighborhood level, we will identify malleable factors for community-level interventions which may be more cost effective and pragmatic than individual-level interventions. At the family level, we will test the direct and indirect contributions of family socio-economic conditions to children's development and identify specific intervention targets for reducing social inequalities in children's mental health. At the individual level, if we discover environmentally-mediated effects between children's subjective social status and their health, the implications would point to children's perceptions of social status as malleable intervention targets and open up a realm of intervention possibilities for empowering children. PUBLIC HEALTH RELEVANCE: About half of the population variation in children's mental health problems is accounted for by environmental factors, and social inequalities stand out as the most prominent determinants of health in the first decade of life. However, the hope of preventing early- emerging mental health problems requires research to identify the underlying mechanisms by which inequalities affect health. Our proposed research tests how social inequalities affect children's health, aiming to identify malleable intervention targets at the community, family, and individual levels.